Chlorine and caustic (sodium or potassium hydroxide) are commercially produced by the electrolysis of sodium or potassium chloride. This can be done in a variety of cell configurations, one of which is a diaphragm cell utilizing a semi-permeable cation exchange membrane. In such case, sodium chloride solution may be provided to the area of an anode which is separated by the membrane from an cathode, to which is supplied pure water. The sodium ions cross the membrane, the chloride ions oxidize at the anode to form chlorine gas, and water is reduced at the anode to form hydrogen and hydroxide gas ions; the hydrogen ions combine to form hydrogen gas, and the hydroxide ions and sodium ions combine to form NaOH, resulting in the net production of sodium hydroxide solution, and hydrogen and chlorine. The ionization of water requires that power be supplied to the electrodes at a sufficient potential (greater than 1.9 V.D.C.) and at the correct current for the desired production rate. Additional process variables include the temperature of the solution, current density on the electrodes, and water and brine flow. The caustic solution is suitable for use directly, although it can be diluted, concentrated or fused dependent on the eventual use. The hydrogen and chlorine are usually compressed, liquified, dried, purified or otherwise treated for convenient handling. These may be either marketed directly or hypochlorite solution may be manufactured by addition of chlorine to the basic solution. The compressed gasses can also be metered to a combustion chamber for the production of hydrochloric acid gas. Usually this is done in a jet with a hydrogen-oxygen pilot flame to insure combustion and a slight (2-5%) excess of hydrogen to insure a chlorine free product. The HCl thus produced then can be absorbed into water to form an aqueous solution, or may be used as a gas. Excess hydrogen can be recycled or mixed with air and vented. Safety equipment must be supplied in the combustion chamber to prevent or vent explosions.
Other methods for the electrolysis of salt or salt solutions are possible, as are alternate methods of hydrochloric acid production. The hydrogen and chlorine can be mixed in equal proportion and brought into contact with incandescent platinum or carbon as a flame initiator and catalyst. Other work has been done using metallic chloride salts such as AlCl.sub.3 or CaCl.sub.2 on a quartz support material as a catalyst at elevated gas temperatures. Pure quartz or alumina can be used at even more elevated temperatures, platinum at lesser temperatures.
Review of caustic-chlorine production can be found in "The Encyclopedia of Chemical Technology", second edition, volume one pp 671-707. Hydrochloric acid production from hydrogen and chlorine is reviewed in volume two, of the same work, page 313. Catalytic methods for hydrogen and chlorine combination are given in "The Explosionless Combustion of Chlorine and Hydrogen to Form Hydrochloric Acid with the Aid of Contact Substances", B. Neumann, Zietschrift for Angewandte Chemie., 34, pp 613-620 (1921).